The cron daemon performs the following tasks at startup:
Checks for new crontab files.
Reads the execution times that are listed within the files.
Submits the commands for execution at the proper times.
Listens for notifications from the crontab commands regarding updated crontab files.
In much the same way, the cron daemon controls the scheduling of at files. These files are stored in the /var/spool/cron/atjobs directory. The cron daemon also listens for notifications from the crontab commands regarding submitted at jobs.